Liquid Crystal Display Device

ABSTRACT

A lighting device including a light source, a light guide plate, a first prism sheet which is arranged over the light guide plate, and a second prism sheet which is arranged between the first prism sheet and the light guide plate. The first prism sheet includes first prisms which are formed parallel to each other in the first direction on a first surface which is opposite to the second prism sheet, and the second prism sheet includes second prisms.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/608,171, filed Oct. 29, 2009, the contents of which are incorporatedherein by reference.

The present application claims priority from Japanese application JP2008-279537 filed on Oct. 30, 2008, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, andmore particularly to a liquid crystal display device which includes abacklight.

2. Description of the Related Art

A liquid crystal display panel is configured such that an envelope isformed of a pair of substrates which sandwiches liquid crystaltherebetween, and optical transmissivity of the liquid crystal displaypanel is controlled over a plurality of pixels which are formed in thespreading direction of the liquid crystal. Accordingly, a liquid crystaldisplay device is usually provided with a backlight which is a planarlight source on a surface (back surface) of the liquid crystal displaypanel on a side opposite to a viewer.

Such a liquid crystal display device is configured to radiate light fromthe backlight through the liquid crystal display panel with a relativelylarge angle with respect to a perpendicular of the liquid crystaldisplay panel. That is, the liquid crystal display device is a liquidcrystal display device having a so-called wide viewing anglecharacteristic which allows a plurality of viewers to observe an imageon the liquid crystal display panel simultaneously.

On the other hand, however, there has been also a demand that, forexample, only a viewer himself is allowed to observe an image on theliquid crystal display panel. For example, as disclosed inJP-A-1994-59287, there has been known a liquid crystal display devicewhich can adjust a viewing angle state from a wide viewing angle stateto a narrow viewing angle state.

The liquid crystal display device disclosed in JP-A-1994-59287 isconfigured such that a guest-host liquid crystal cell is arrangedbetween a backlight and a liquid crystal display panel and hence, whenthe guest-host liquid crystal cell is driven, light from the backlightcan be radiated with a small angle with respect to a perpendicular ofthe liquid crystal display panel. The guest-host cell per se can adjusta viewing angle of light which passes through the guest-host liquidcell.

SUMMARY OF THE INVENTION

However, the liquid crystal display device disclosed in JP-A-1994-59287is configured such that the guest-host liquid crystal cell having arelatively large thickness is interposed between the liquid crystaldisplay panel and the backlight. Accordingly, there arises a drawbackthat a thickness of the liquid crystal display device is largelyincreased. Further, the guest-host liquid crystal cell has therelatively complicated configuration thus giving rise to a drawback thata manufacturing cost of the liquid crystal display device is pushed up.Further, the adjustment of a viewing angle of the guest-host liquidcrystal cell is performed by applying a voltage to the guest-host liquidcrystal cell and hence, there arises a drawback that the increase ofpower consumption is unavoidable.

It is an object of the invention to provide a liquid crystal displaydevice which can adjust a viewing angle with the simple configurationwithout incurring the large increase of a thickness and the increase ofpower consumption.

The liquid crystal display device of the invention is configured suchthat, for example, two prism sheets are arranged between a liquidcrystal display panel and a light guide plate in an overlapping manner,and the prism sheet on a liquid-crystal-display-panel side (first prismsheet) is displaced thus performing a control on whether light which isradiated from the light guide plate and is incident through the prismsheet on a light-guide-plate side (second prism sheet) is allowed to bereflected on the first prism sheet or to pass through the first prismsheet directly so as to adjust a viewing angle.

Further, the liquid crystal display device of the invention may use oneprism sheet corresponding to the first prism sheet in place of two prismsheets and may make the light guide plate also play a role of the secondprism sheet.

The liquid crystal display device of the invention can be configured asfollows, for example.

(1) According to one aspect of the invention, there is provided a liquidcrystal display device which includes: a liquid crystal display panel;and a backlight which is arranged on a back surface of the liquidcrystal display panel, wherein

the backlight includes a light source, a light guide plate, a firstprism sheet which is arranged between the liquid crystal display paneland the light guide plate, and a second prism sheet which is arrangedbetween the first prism sheet and the light guide plate,

the first prism sheet includes first prisms which are formed parallel toeach other in the first direction on a liquid-crystal-display-panel-sidesurface thereof,

the first prism formed on the first prism sheet includes a firstinclined surface and a second inclined surface which respectively extendin the second direction which intersects the first direction,

assuming light which is emitted from the light source, enters the lightguide plate through a side surface of the light guide plate, and isradiated from an upper surface of the light guide plate as a firstlight, and light which enters the light guide plate through the uppersurface of the light guide plate and is radiated from the upper surfaceof the light guide plate as a second light,

the second prism sheet includes second prisms which convert the firstlight into a third light whose light quantity has the distribution wherea magnitude of light quantity is periodically changed at a pitch of thefirst prisms formed on the first prism sheet in the first direction andallow the third light to be incident on the first prism sheet,

the first prism sheet, upon impingement of the third light on the firstinclined surfaces, reflects the third light toward asecond-inclined-surface side on the first inclined surfaces thusreflecting at least a portion of the third light toward alight-guide-plate side on the second inclined surface,

the first prism sheet, upon impingement of the third light on the secondinclined surfaces without via the first inclined surfaces, radiates thethird light toward a liquid-crystal-display-panel side, and

the first prism sheet is displaceable between a first state in a wideviewing angle mode where a rate that the third light impinges on thesecond inclined surfaces without via the first inclined surfaces issmaller than a rate that the third light impinges on the first inclinedsurface and a second state in a narrow viewing angle where the rate thatthe third light impinges on the second inclined surfaces without via thefirst inclined surfaces is larger than the rate that the third lightimpinges on the first inclined surface.

(2) In the liquid crystal display device having the configuration (1),the displacement of the first prism sheet may be performed in the firstdirection.

(3) In the liquid crystal display device having the configuration (1),the displacement of the first prism sheet may be performed in thedirection perpendicular to a main surface of the first prism sheet.

(4) In the liquid crystal display device having the configuration (1),the plurality of second prisms which are formed parallel to each otherin the first direction may be formed on a first-prism-sheet-side surfaceof the second prism sheet,

the second prism formed on the second prism sheet may include a thirdinclined surface and a fourth inclined surface which respectively extendin the second direction, and

the third light may be radiated from the fourth inclined surfaces.

(5) In the liquid crystal display device having the configuration (1),the second light may be a diffused light.

(6) In the liquid crystal display device having the configuration (1),the liquid crystal display panel may include a pair of substrates whichsandwiches liquid crystal therebetween, and the liquid crystal may bedriven by an electric field which has components parallel to the pair ofsubstrates.

(7) According to another aspect of the invention, there is provided aliquid crystal display device which includes: a liquid crystal displaypanel; and a backlight which is arranged on a back surface of the liquidcrystal display panel, wherein

the backlight includes a light source, a light guide plate, a prismsheet which is arranged between the liquid crystal display panel and thelight guide plate,

the prism sheet includes first prisms which are formed parallel to eachother in the first direction on a liquid-crystal-display-panel-sidesurface thereof,

the first prism formed on the prism sheet includes a first inclinedsurface and a second inclined surface which respectively extend in thesecond direction which intersects the first direction,

assuming light which is emitted from the light source, enters the lightguide plate through a side surface of the light guide plate, and isradiated from an upper surface of the light guide plate as a firstlight, and light which enters the light guide plate through the uppersurface of the light guide plate and is radiated from the upper surfaceof the light guide plate as a second light,

the light guide plate includes, on a lower surface thereof, secondprisms which radiate the first light from an upper surface of the lightguide plate such that the first light turns into a light whose lightquantity has the distribution where a magnitude of light quantity isperiodically changed at a pitch of the first prisms formed on the prismsheet in the first direction,

the prism sheet, upon impingement of the first light on the firstinclined surfaces, reflects the first light toward asecond-inclined-surface side on the first inclined surfaces and reflectsat least a portion of the first light toward a light-guide-plate side onthe second inclined surface,

the prism sheet, upon impingement of the first light on the secondinclined surfaces without via the first inclined surfaces, radiates thefirst light toward a liquid-crystal-display-panel side, and

the prism sheet is displaceable between a first state in a wide viewingangle mode where a rate that the first light impinges on the secondinclined surfaces without via the first inclined surfaces is smallerthan a rate that the first light impinges on the first inclined surfaceand a second state in a narrow viewing angle mode where the rate thatthe first light impinges on the second inclined surfaces without via thefirst inclined surfaces is larger than the rate that the first lightimpinges on the first inclined surface.

(8) In the liquid crystal display device having the configuration (7),the displacement of the prism sheet may be performed in the firstdirection.

(9) In the liquid crystal display device having the configuration (7),the displacement of the prism sheet may be performed in the directionperpendicular to a main surface of the prism sheet.

(10) In the liquid crystal display device having the configuration (7),a pitch of the second prisms may be equal to a pitch of the firstprisms.

(11) In the liquid crystal display device having the configuration (7),the second light may be a diffused light.

(12) In the liquid crystal display device having the configuration (7),the liquid crystal display panel includes a pair of substrates whichsandwiches liquid crystal therebetween, and the liquid crystal is drivenby an electric field which has components parallel to the pair ofsubstrates.

(13) According to still another aspect of the invention, there isprovided a liquid crystal display device which includes: a liquidcrystal display panel; and a backlight which is arranged on a backsurface of the liquid crystal display panel, wherein the backlightincludes a plurality of optical sheets, and at least one optical sheetis displaced so as to adjust a viewing angle.

(14) According to still another aspect of the invention, there isprovided a liquid crystal display device which includes: a liquidcrystal display panel; and a backlight which is arranged on a backsurface of the liquid crystal display panel, wherein the backlightincludes a plurality of optical sheets, and at least one optical sheetis moved so as to adjust a viewing angle.

(15) In the liquid crystal display device having the configuration (13),the plurality of optical sheets may be formed of a prism sheetrespectively.

(16) In the liquid crystal display device having the configuration (14),the plurality of optical sheets may be formed of a prism sheetrespectively.

(17) In the liquid crystal display device having the configuration (13),the displacement may be performed in the horizontal direction.

(18) In the liquid crystal display device having the configuration (13),the displacement may be performed in the vertical direction.

(19) In the liquid crystal display device having the configuration (14),the movement may be performed in the horizontal direction.

(20) In the liquid crystal display device having the configuration (14),the movement may be performed in the vertical direction.

The above-mentioned configurations are merely examples, and theinvention can be suitably modified without departing from the technicalconcept of the invention. Further, configurational examples of theinvention other than the above-mentioned configurational examples willbecome apparent from the description of the whole specification andattached drawings.

The liquid crystal display device having the above-mentionedconfigurations can adjust a viewing angle in spite of the simpleconfiguration thereof.

Other advantageous effects of the invention will become apparent fromthe description of the whole specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing the configuration of an essential part of aliquid crystal display device according to an embodiment 1 of theinvention and also is a view showing a state where a path of light froma light guide plate is changed along with the displacement of a firstprism sheet;

FIG. 1B is a view showing the configuration of an essential part of theliquid crystal display device according to the embodiment 1 of theinvention and also is a view showing a state where a path of light froma light guide plate is changed along with the displacement of the firstprism sheet;

FIG. 2A is a plan view showing the overall configuration of the liquidcrystal display device according to the embodiment 1 of the invention;

FIG. 2B is a cross-sectional view showing the overall configuration ofthe liquid crystal display device according to the embodiment 1 of theinvention;

FIG. 3 is a view showing the distribution of magnitude of a lightradiated from a second prism sheet of the liquid crystal display deviceof the invention;

FIG. 4 is a viewing angle characteristic graph showing an advantageouseffect brought about by the liquid crystal display device according tothe embodiment 1 of the invention;

FIG. 5A is a view showing the configuration of a liquid crystal displaydevice according to an embodiment 2 of the invention;

FIG. 5B is a view showing the configuration of the liquid crystaldisplay device according to the embodiment 2 of the invention;

FIG. 6A is a view showing the configuration of a liquid crystal displaydevice according to an embodiment 4 of the invention; and

FIG. 6B is a view showing the configuration of the liquid crystaldisplay device according to the embodiment 4 of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention are explained in conjunction with drawingshereinafter. Here, in all drawings and the respective embodiments,identical or similar configurational parts are given same symbols andtheir repeated explanation is omitted.

Embodiment 1

FIG. 2A and FIG. 2B are views showing the configuration of a liquidcrystal display device according to the embodiment 1 of the invention,wherein FIG. 2A is a plan view of the liquid crystal display device inan exploded state, and FIG. 2B is a cross-sectional view of the liquidcrystal display device in a non-exploded state taken along a line b-b inFIG. 2A.

In FIG. 2, the liquid crystal display device is constituted of at leasta liquid crystal display panel PNL, a first prism sheet PS1, a secondprism sheet PS2, and a light guide plate CLB which are arranged in orderfrom a viewer's side.

The liquid crystal display panel PNL includes an envelope formed ofsubstrates SUB1, SUB2 which are arranged to face each other in anopposed manner with liquid crystal sandwiched therebetween (see FIG.2B), and an image display part AR which is constituted of a plurality ofpixels is incorporated in the envelope. Further, liquid crystal issealed between the substrates SUB1, SUB2 by a sealing material SL whichis formed in a state that the sealing material SL surrounds the imagedisplay part AR. The sealing material SL also adheres the substratesSUB1, SUB2 to each other. The liquid crystal display panel PNL is formedof, for example, a so-called IPS (In Plane Switching) type panel or aso-called lateral-electric-field-type panel, wherein an electric fieldapplied to liquid crystal in each pixel has components parallel to thesubstrates. The liquid crystal display panel PNL having such aconfiguration exhibits an excellent wide viewing angle characteristic.Here, although not shown in the drawing, the liquid crystal displaypanel PNL includes a polarizer arranged on a front face surface side(viewer's side) thereof and a polarizer arranged on a back surface side(backlight BL side) thereof respectively.

In this embodiment, the backlight BL is arranged on a back surface sideof the liquid crystal display panel PNL, and includes at least a lightsource LS, a light guide plate CLB, a first prism sheet PS1 and a secondprism sheet PS2.

A plurality of prisms (first prisms) which are arranged parallel to eachother in the x direction in the drawing are formed on aliquid-crystal-display-panel-PNL-side surface of the first prism sheetPS1. The plurality of prisms have a triangular shape in which an apexangle is made by a pair of inclined surfaces which extend in the ydirection in the drawing. A partially enlarged view A1 shown in FIG. 2Ashows an enlarged portion of the prism sheet PS1 in a dotted frame a1,and a partially enlarged view B1 shown in FIG. 2B shows an enlargedportion of the prism sheet PS1 in a dotted frame b1. Both the enlargedviews A1, B1 show the above-mentioned prisms formed on the first prismsheet PS1. Here, as shown in the enlarged views A1 and B1, out of thepair of inclined surfaces which forms the apex angle SM1 of the prism,one inclined surface (for example, inclined surface on a left side ofthe drawing) is referred to as a first inclined surface SLP11, andanother inclined surface (for example, inclined surface on a right sideof the drawing) is referred to as a second inclined surface SLP12. Thisdefinition is made to ease the explanation made later. Further, theabove-mentioned liquid crystal display panel PNL, the second prism sheetPS2 and the light guide plate CLB described later respectively assumethe positional relationship which is fixed relative to each other, whilethe first prism sheet PS1 is configured to perform the fine displacementin the x direction as indicated by an arrow P in the drawing.

A plurality of prisms (second prisms) which are arranged parallel toeach other in the x direction in the drawing are formed on afirst-prism-sheet-PS1-side surface of the second prism sheet PS2. Theplurality of prisms have a triangular shape in which an apex angle ismade by a pair of inclined surfaces which extends in the y direction inthe drawing. A partially enlarged view A2 shown in FIG. 2A shows anenlarged portion of the prism sheet PS2 in a dotted frame a2, and apartially enlarged view B2 shown in FIG. 2B shows an enlarged portion ofthe prism sheet PS2 in a dotted frame b2. Both the enlarged views A2, B2show the above-mentioned prisms formed on the second prism sheet PS2.Here, as shown in the enlarged views A2 and B2, out of the pair ofinclined surfaces which forms the apex angle SM2 of the prism, oneinclined surface (for example, inclined surface on a left side of thedrawing) is referred to as a first inclined surface SLP21, and anotherinclined surface (for example, inclined surface on a right side of thedrawing) is referred to as a second inclined surface SLP22. Thisdefinition is made to ease the explanation made later. The second prismsheet PS2 is configured to function so as to guide a light from thelight guide plate CLB toward a first-prism-sheet-PS1 side.

The backlight BL includes the light guide plate CLB which is arranged toface at least the image display part AR of the liquid crystal displaypanel PNL in an opposed manner and the light source LS which is arrangedto face a side wall surface of the light guide plate CLB in an opposedmanner. The light source LS is arranged to face the side wall surfaceSWF formed on a side of the light guide plate CLB parallel to the ydirection in the drawing. Due to such a configuration, a light from thelight source LS enters the light guide plate CLB through the side wallsurface (side surface) SWF, is reflected on a lower surface (a surfaceon a side opposite to the second prism sheet PS2) of the light guideplate CLB, for example, and, thereafter, is radiated from an uppersurface (a surface on a second-prism-sheet-PS2 side) of the light guideplate CLB. In this case, the light which is radiated from thesecond-prism-sheet-PS2-side surface of the light guide plate CLB isradiated at small angle with respect to a surface of the light guideplate CLB and hence, the light is made to pass through the second prismsheet PS2 so as to refract the light toward the first-prism-sheet-PS1side. Here, the light source LS may be a linear light source such as afluorescent lamp or a spot light source such as an LED.

In the liquid crystal display device having such a configuration, FIG.1A and FIG. 1B are views showing a state where a path of light from thelight guide plate CLB is changed along with the displacement of a firstprism sheet PS1.

Both FIG. 1A and FIG. 1B show the light guide plate CLB, the secondprism sheet PS2, and the first prism sheet PS1. The difference betweenthe configuration shown in FIG. 1A and the configuration shown in FIG.1B lies in that, compared to the first prism sheet PS1 shown in FIG. 1A,the first prism sheet PS1 shown in FIG. 1B is displaced by a distanceindicated by DH in the drawing, for example. This displacement isgenerated due to the movement of the first prism sheet PS1 along the xdirection in the drawing in FIG. 2A.

FIG. 1A is a view for explaining a state of the liquid crystal displaypanel PNL in a wide viewing angle mode. Firstly, as shown in FIG. 1A,although not shown in the drawing, a light emitted from the light sourceLS enters the light guide plate CLB through the side wall surface SWF,is reflected on a surface of the light guide plate CLB on a sideopposite to the second prism sheet PS2, for example, and, thereafter, isradiated from the second-prism-sheet-PS2-side surface of the light guideplate CLB. Here, to facilitate the understanding of the invention, thelight which is emitted from the light source LS, enters the light guideplate CLB through the side surface of the light guide plate CLB, and isradiated from the upper surface of the light guide plate CLB is referredto as the first light. Here, the first light is radiated at a relativelysmall angle θ with respect to the front surface of the light guide plateCLB. Here, to differentiate the light which enters the light guide plateCLB through the upper surface of the light guide plate CLB, is reflectedon a lower surface of the light guide plate CLB and is radiated from theupper surface of the light guide plate CLB again from the first light,such a light is referred to as the second light.

Next, the first light radiated from the light guide plate CLB isincident on the second prism sheet PS2, and most of the first light isradiated from the second inclined surfaces SLP22 which constitute theprisms. This is because, different from the first inclined surfacesSLP21, the perpendicular direction of the second inclined surfaces SLP22are arranged to be substantially aligned with an optical path of thefirst light after the incidence on the second prism sheet PS2. However,the perpendicular direction of the second inclined surfaces SLP22 andthe optical path of the first light may not be completely aligned, and aslight inclination may be imparted to the second inclined surfaces SLP22so as to adjust an angle of the light radiated from the second prismsheet PS2. In this manner, by allowing the light radiated from thesecond-prism-sheet-PS2-side surface of the light guide plate CLB to passthrough the second prism sheet PS2, the light refracted in the inside ofthe second prism sheet PS2 is sufficiently directed toward thefirst-prism-sheet-PS1 side. Here, the first inclined surfaces SLP21 arearranged to be substantially aligned with the optical path of the firstlight after the incidence on the second prism sheet PS2 and hence, thereis almost no possibility that the first light is radiated from the firstinclined surfaces SLP21.

In this case, as described above, most of the light radiated from thesecond prism sheet PS2 is radiated from the second inclined surfacesSLP22 and hence, the light radiated from the second prism sheet PS2 isdirected toward the first-prism-sheet-PS1 side with the distribution ofoptical quantity where the magnitude of optical quantity is periodicallychanged in the x direction shown in FIG. 2A. That is, as shown in FIG.3, in a region TR corresponding to the front surface of the second prismsheet PS2, the optical quantity is large in regions TR2 where the secondinclined surface SLP22 is formed, and the optical quantity is small inregions TR1 where the first inclined surface SLP21 is formed.

Accordingly, the second prisms formed on the second prism sheet PS2 hasa function of converting the first light into a third light with thedistribution of optical quantity where the magnitude of optical quantityis periodically repeated at a pitch of the first prisms formed on thefirst prism sheet PS1 along the predetermined direction (the directionin which the first prisms of the first prism sheet PS1 are arrangedparallel to each other, that is, the x direction in the drawing) andallowing the first light to be incident on the first prism sheet PS1.

Then, the third light radiated from the second prism sheet PS2 isincident on the first prism sheet PS1. Most of the third light (largerthan a half of the third light) which is incident on the first prismsheet PS1 is configured to be incident on the first inclined surfacesSLP11 which constitute the prisms when the first prism sheet PS1 is at aposition shown in FIG. 1A. Most of the light which is incident on thefirst inclined surface SLP11 is reflected on the first inclined surfaceSLP11 and is incident on the second inclined surface SLP12. Most of thelight which is incident on the second inclined surfaces SLP12 isdirected toward a second-prism-sheet-PS2-side surface of the first prismsheet PS1, and is incident on the second prism sheet PS2. Although anoptical path which follows thereafter is not illustrated in the drawing,the light is directed toward a liquid crystal display panel PNL sidethrough a relatively complicated path such that, for example, the lightenters the light guide plate CLB again through the upper surface of thelight guide plate CLB, is reflected on a lower surface of the lightguide plate CLB, and is radiated from the upper surface of the lightguide plate CLB again. That is, the light is converted into the secondlight. The second light is, different from the light having highdirectivity such as the first light, a diffusion light having componentsdirected in various directions. Accordingly, although the second lightpasses through the first prism sheet PS1 and the second prism sheet PS2,the second light is incident on the first prism sheet PS1 and the secondprism sheet PS2 at angles different from an incident angle of the firstlight and hence, the second light is radiated from the first prism sheetPS1 and the second prism sheet PS2 at various angles so that,eventually, the second light is incident on the liquid crystal displaypanel PNL at various angles.

Although a portion of light which is reflected on the first inclinedsurfaces SLP11 may be radiated from the first prism sheet PS1 withoutbeing reflected on the second inclined surfaces SLP12, this light isalso reflected on various portions and is incident on the liquid crystaldisplay panel PNL at various angles.

Accordingly, light directed toward the liquid crystal display panel PNLis constituted of lights incident from a large number of directions, anda viewing angle characteristic of the liquid crystal display panel PNLis determined based on properties of the liquid crystal display panelPNL. The liquid crystal display panel PNL is, as described above, formedof a panel having the excellent wide-viewing-angle characteristic andhence, the liquid crystal display panel PNL can acquire a wide viewingangle.

Next, FIG. 1B is a view for explaining a state of the liquid crystaldisplay panel PNL in a narrow viewing angle mode. In FIG. 1B, the firstprism sheet PS1 is displaced with respect to the first prism sheet PS1shown in FIG. 1A by a distance indicated by DH in the drawing. Anoptical path along which light emitted from the light source LS isincident on the second prism sheet PS2 as the first light and isradiated as the third light is substantially equal to the correspondingoptical path shown in FIG. 1A. Then, most of the third light (at leastlarger than a half of the third light) which is incident on the firstprism sheet PS1 from the second prism sheet PS2 is incident on thesecond inclined surfaces SLP12 without via the first inclined surfacesSLP11. The light which is incident on the first inclined surfaces SLP11is, based on the law of refraction, radiated in the directionsubstantially perpendicular to the surface of the liquid crystal displaypanel PNL (not shown in the drawing) (desirably in the direction whichmakes 20 degrees or less with respect to the perpendicular direction).The light which is incident on the liquid crystal display panel PNL hasdirectivity in this manner and hence, an image formed by the liquidcrystal display panel PNL is displayed as an image having a narrowviewing angle.

It is considered that a portion of the third light impinges on the firstinclined surfaces SLP11 and traces the same path taken in the wideviewing angle mode which is explained in conjunction with FIG. 1A.However, a rate of such light is small and hence, visibility when theliquid crystal display panel PNL is observed from the oblique directionbecomes poor so that a narrow viewing angle can be achieved.

FIG. 4 is a viewing angle characteristic graph showing an advantageouseffect brought about by the liquid crystal display device according tothe embodiment 1 of the invention. In FIG. 4, a viewing angle φ is takenon an axis of abscissas and optical intensity IL of a display of theliquid crystal display device is taken on an axis of ordinates. Assumethat the viewing angle φ becomes 0° when the liquid crystal displaypanel PNL is viewed from the perpendicular direction. A characteristic αin the drawing is a characteristic obtained by arranging the first prismsheet PS1 as shown in FIG. 1A. It is understood from the characteristicα that the liquid crystal display panel PNL is excellent in theso-called wide viewing angle where lowering of optical intensity IL issmall even when a viewing angle is large. On the other hand, acharacteristic β in the drawing is a characteristic obtained byarranging the first prism sheet PS1 as shown in FIG. 1B. It isunderstood from the characteristic β that the liquid crystal displaypanel PNL is excellent in the so-called narrow viewing angle whereoptical intensity IL is large only when a viewer observes an image at anangle substantially perpendicular to the display screen and is sharplydecreased in a midst of the increase of a viewing angle.

The above-mentioned displacement of the first prism sheet PS1 is broughtabout by a switching means not shown in the drawing which is manuallyoperated, for example. When an operation mode is changed over to onemode out of two modes, for example, by operating the switching means,the first prism sheet PS1 may take the arrangement shown in FIG. 1A, andwhen the operation mode is changed over to the other mode, the firstprism sheet PS1 may take the arrangement shown in FIG. 1B. Further, inthe above-mentioned explanation, the first prism sheet PS1 is displacedrelative to the second prism sheet PS2 by a distance DH (correspondingto approximately half pitch of the prisms). However, an amount ofdisplacement is not limited to the distance DH, and may be odd times aslarge as the distance DH. In other words, it is sufficient that thearrangement of the first prism sheet PS1 relative to the second prismsheet PS2 assume at least the state shown in FIG. 1A and the state shownin FIG. 1B. That is, it is sufficient that the first prism sheet PS1 isdisplaceable between a first state in the wide viewing angle mode wherea rate that the third light impinges on the second inclined surfacesSLP12 without via the first inclined surfaces SLP11 is smaller than arate that the third light impinges on the first inclined surface SLP11and a second state in a narrow viewing angle mode where the rate thatthe third light impinges on the second inclined surfaces SLP12 withoutvia the first inclined surfaces SLP11 is larger than the rate that thethird light impinges on the first inclined surfaces SLP11.

According to the liquid crystal display device having such aconfiguration, it is possible to change over the viewing angle betweenthe wide viewing angle and the narrow viewing angle by displacing ormoving the first prism sheet PS1 and hence, the configuration of theliquid crystal display device can be simplified. Further, it issufficient to interpose the first prism sheet PS1 and the second prismsheet PS2 between the liquid crystal display panel PNL and the lightguide plate CLB and hence, it is no more necessary to largely increase athickness of the liquid crystal display device. Further, it isunnecessary to use an element such as a guest-host liquid crystal celland hence, it is possible to obviate the increase of power consumption.

Embodiment 2

FIG. 5A and FIG. 5B are views showing the configuration of a liquidcrystal display device according to an embodiment 2 of the invention,and are drawn corresponding to FIG. 1A and FIG. 1B.

The configuration of the liquid crystal display device shown in FIG. 5Aand FIG. 5B differs from the configuration of the liquid crystal displaydevice shown in FIG. 1A and FIG. 1B with respect to a point that thefirst prism sheet PS1 is displaced in the vertical direction withrespect to a main surface of the first prism sheet PS1. That is, in FIG.5B, the first prism sheet PS1 is displaced in the z direction in thedrawing from a position shown in FIG. 5A (indicated by a dotted line inFIG. 5B) by a distance DV in the drawing.

Also in such a case, most of the third light which is incident on thefirst prism sheet PS1 from the second prism sheet PS2 in FIG. 5B isincident on the second inclined surfaces SLP12 of the first prism sheetPS1 without via the first inclined surfaces SLP11 of the first prismsheet PS1 and hence, this embodiment 2 can acquire the substantiallysame advantageous effects as the embodiment 1.

However, an amount of displacement of the first prism sheet PS1 relativeto the second prism sheet PS2 is not limited to the distance DV. It issufficient that the arrangement of the first prism sheet PS1 relative tothe second prism sheet PS2 assume at least the state shown in FIG. 5Aand the state shown in FIG. 5B. That is, it is sufficient that the firstprism sheet PS1 is displaceable between a first state in the wideviewing angle mode where a rate that the third light impinges on thesecond inclined surfaces SLP12 without via the first inclined surfacesSLP11 is smaller than a rate that the third light impinges on the firstinclined surface SLP11 and a second state in a narrow viewing angle modewhere the rate that the third light impinges on the second inclinedsurfaces SLP12 without via the first inclined surfaces SLP11 is largerthan the rate that the third light impinges on the first inclinedsurfaces SLP11.

Embodiment 3

In the above-mentioned embodiments, the configuration for acquiring thewide viewing angle and the configuration for acquiring the narrowviewing angle are exemplified. In the above-mentioned embodiments, theswitching means which changes over these configurations is alsoexemplified. However, the invention is not limited to suchconfigurations and switching means. That is, the displacement of thefirst prism sheet may be performed such that the first prism sheet maybe displaced in three or more stages which differ in width of viewingangle from each other using the above-mentioned switching means.Further, the first prism sheet may be displaced continuously withoutlimiting the number of stages. All these cases, however, include atleast the wide viewing angle mode and the narrow viewing angle modeexplained in conjunction with the embodiments 1, 2.

Embodiment 4

FIG. 6A and FIG. 6B are views showing the configuration of a liquidcrystal display device according to an embodiment 4 of the invention,and are drawn corresponding to FIG. 1A and FIG. 1B.

In the embodiment 4, the liquid crystal display device does not includethe second prism sheet PS2. Instead, projecting portions PR which extendin the x direction in FIG. 2A and are arranged parallel to each other inthe y direction are formed on a lower surface of the light guide plateCLB (a surface of the light guide plate CLB on a side opposite to thefirst prism sheet PS1), and a pitch of the projecting portions PR is setequal to a pitch of the prisms formed on the first prism sheet PS1. Theprojecting portions PR have the same function as prisms. That is, theprojecting portions PR reflect the light which is emitted from the lightsource LS and enters the light guide plate CLB through the side surfaceof the light guide plate CLB, and radiate the light toward the firstprism sheet PS1 side from the upper surface of the light guide plate CLBas the first light.

The first light acquires the distribution of magnitude of light shown inFIG. 3 on the first-prism-sheet-PS1-side surface of the light guideplate CLB. Accordingly, the projecting portions PR formed on the lightguide plate CLB have the similar function as the second prism sheet PS2.That is, the first light in the embodiment 4 is in the same stage as thethird light in the embodiment 1. The explanation of an optical path inthe inside of the first prism sheet PS1 of this embodiment 4 issubstantially equal to the optical path of the third light in theembodiment 1 except for that the third light in the embodiment 1 isreplaced with the first light in the embodiment 4. Accordingly, thisembodiment 4 can acquire the substantially equal advantageous effects asthe embodiment 1. The embodiment 4 may be applicable to the embodiment 2or 3.

Embodiment 5

The above-mentioned embodiments adopt the so-calledlateral-electric-field-type liquid crystal display panel or theso-called IPS-type liquid crystal display panel as the liquid crystaldisplay panel PNL. However, the invention is not limited to thesepanels, and a so-called vertical-electric-field-type liquid crystaldisplay panel such as a TN (Twisted Nematic)-type liquid crystal displaypanel or a VA-type liquid crystal display panel may be used as theliquid crystal display panel PNL.

Although the invention has been explained in conjunction with theembodiments, the configurations explained with respect to the respectiveembodiments are merely examples and the invention may be suitablymodified without departing from the technical concept of the invention.Further, the configurations explained with respect to the respectiveembodiments may be combined to each other unless these embodimentscontradict each other.

What is claimed is:
 1. A lighting device comprising: a light source; alight guide plate; a first prism sheet which is arranged over the lightguide plate; and a second prism sheet which is arranged between thefirst prism sheet and the light guide plate, wherein the first prismsheet includes first prisms which are formed parallel to each other inthe first direction on a first surface which is opposite to the secondprism sheet; the first prism formed on the first prism sheet includes afirst inclined surface and a second inclined surface which respectivelyextend in the second direction which intersects the first direction;light which is emitted from the light source, enters the light guideplate through a side surface of the light guide plate, and is radiatedfrom an upper surface of the light guide plate as a first light, andlight which enters the light guide plate through the upper surface ofthe light guide plate and is radiated from the upper surface of thelight guide plate as a second light; the second prism sheet includessecond prisms which convert the first light into a third light whoselight quantity has the distribution where a magnitude of light quantityis periodically changed at a pitch of the first prisms formed on thefirst prism sheet in the first direction and allow the third light to beincident on the first prism sheet; the first prism sheet, uponimpingement of the third light on the first inclined surfaces, reflectsthe third light toward a second-inclined-surface side on the firstinclined surfaces thus reflecting at least a portion of the third lighttoward a light-guide-plate side on the second inclined surface; thefirst prism sheet, upon impingement of the third light on the secondinclined surfaces without via the first inclined surfaces, radiates thethird light toward the first surface; and the first prism sheet isdisplaceable between a first state in a wide viewing angle mode where arate that the third light impinges on the second inclined surfaceswithout via the first inclined surfaces is smaller than a rate that thethird light impinges on the first inclined surface and a second state ina narrow viewing angle where the rate that the third light impinges onthe second inclined surfaces without via the first inclined surfaces islarger than the rate that the third light impinges on the first inclinedsurfaces.
 2. A lighting device according to claim 1, wherein adisplacement of the first prism sheet is performed in the firstdirection.
 3. A lighting device according to claim 1, wherein adisplacement of the first prism sheet is performed in the directionperpendicular to a main surface of the first prism sheet.
 4. A lightingdevice according to claim 1, wherein the plurality of second prismswhich are formed parallel to each other in the first direction areformed on a first-prism-sheet-side surface of the second prism sheet;the second prism formed on the second prism sheet includes a thirdinclined surface and a fourth inclined surface which respectively extendin the second direction; and the third light is radiated from the fourthinclined surfaces.
 5. A lighting device according to claim 1, whereinthe second light is a diffused light.
 6. A liquid crystal display devicecomprising a lighting device according to claim 1, wherein the liquidcrystal display device includes a liquid crystal display panel includinga pair of substrates which sandwiches liquid crystal therebetween, andthe liquid crystal is driven by an electric field which has componentsparallel to the pair of substrates.